1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery that achieves high capacity and to a method of manufacturing the battery.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, non-aqueous electrolyte secondary batteries, which perform charge and discharge by transferring lithium ions between the positive and negative electrodes, have been widely used as a driving power source for the mobile information terminal devices.
As the mobile information terminal devices tend towards having greater numbers of functions, such as moving picture playing functions and gaming functions, the power consumption of the devices tends to increase. It is therefore strongly desired that the non-aqueous electrolyte secondary batteries used for the power sources of such devices have further higher capacities and higher performance to achieve longer battery life and improved output power. In addition, applications of the non-aqueous electrolyte secondary batteries are expected to expand from just the above-described applications but to power tools, power assisted bicycles, and moreover hybrid electric vehicles (HEVs) and electric vehicles (EVs). In order to meet such expectations, it is strongly desired that the capacity and the performance of the battery be improved further.
In order to increase the capacity of the non-aqueous electrolyte secondary battery, it is necessary to increase the capacity of the positive electrode. In particular, layered compounds are viewed as promising materials for positive electrode active materials. To date, many lithium-containing layered compounds have been studied. Among the materials that have been developed are LiCoO2, LiNiO2, LiNi1/3Co1/3Mn1/3O2, and NaxCoyMn1-yO2 where 0.6≦x≦0.8 and 0.4≦y≦0.6 (see Japanese Published Unexamined Patent Application No. 2002-220231).
In addition, a technique for synthesizing a lithium compound from a sodium compound has been studied as a method for synthesizing a novel lithium compound (see Japanese Published Unexamined Patent Application No. 2007-220650). According to this method a layered compound, which is difficult to synthesize with lithium, can be easily obtained. In particular, Na0.7CoO2 and NaCo1/2Mn1/2O2 can be used as positive electrode active materials for lithium-ion batteries by ion-exchanging sodium for lithium. Therefore, much research has been conducted on synthesis methods and ion-exchange methods by chemical techniques using Na0.7CoO2 and NaCo1/2Mn1/2O2.
The positive electrode active materials using sodium-based oxides are promising materials that are expected to yield high capacity, and by adding lithium thereto, further high capacity can be obtained. However, the addition of lithium causes the average discharge potential to decrease. Moreover, it causes formation of an impurity similar to Li2MnO3, resulting in the problem of side reactions during charge and discharge.